Investigate the movement of water into and out of plant cells by osmosis.

GCSE Science

Potato Cores in Salt Solution

Scenario

Plants in the soil have their roots in a dilute solution of mineral ions. When they are suddenly

flooded with seawater, concentration of which is 0.3 molar Sodium Chloride, they are observed

to wilt and become flaccid.

Aim

The aim of this experiment is to investigate the movement of water into and out of plant cells by

osmosis. The cells chosen for study will be taken from potato tubers as they provide a ready

supply of uniform material.

Background Information

Any substance dissolved in water is called a solute; a solvent is a liquid that is able to dissolve

another substance, called a solute, to form a solution.

The water content of plants varies depending on environmental conditions. In land plants it plays

a vital role in structural support and mineral transport and thus a lack of water may lead to

wilting or possibly death.

Water is mainly absorbed through the roots, which are covered in specially adapted root hair

cells, with large surface areas and thin cell walls to aid absorption by osmosis. The evaporation

of water through stomata on plant leaves causes a transpiration stream, causing the water to be

drawn up through xylem vessels.

Osmosis is the flow of water molecules by diffusion through a partially permeable membrane

from areas of high water potential (low solute concentrations) to regions of low water potential

(high solute concentrations).

All plant cell membranes are partially permeable, which means they allow some some

substances to penetrate them but not others.

Whether water enters the cell by osmosis will depend on the balance between external and

internal solute and water potentials. If the solutions on each side of the partially permeable

membrane are of equal water or solute potential, then there will be no net movement of water

molecules across the membrane. This is called an equilibrium state and the solutions are referred

to as being isotonic.

A solution that contains more solute particles than another, and hence has a low water potential,

is referred to as being hypertonic, whilst the less concentrated solution is hypotonic.

The concentration of solute particles is described as a molarity. One mole of any substance is

the mass of 6.02 x 1023 particles of the substance. The molarity of a solution can be calculated

using the below equation:

Molarity = Moles of Solute

Litres of Solution

If a plant was exposed to a waterlogged environment, with the external solute concentration to

the cell being hypotonic to the vacuole contents, the cell will not continue to take in water by

osmosis forever. The cellulose wall provides a rigid barrier to uncontrolled expansion. A cell

that is full of water is called turgid and cannot expand further as the inward force of the starched

wall balances the outward pressure on the cell contents. This wall pressure is called turgor

pressure and the internal outward force on the wall is called the osmotic pressure.

At the other extreme, a cell placed in a solution that is hypertonic to its contents will lose water

molecules by osmosis. The cytoplasm will cease to exert a pressure on the cellulose cell wall

and the cell, described as flaccid, will lack support. Water loss can continue to such an extent

that the cytoplasm, and attached cell membrane, contracts and detaches from the cell wall. A

cell in this condition is said to be plasmolysed and this damage is irreversible.

Safety notes

· Use care when working with glassware.

· Wash your ...

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At the other extreme, a cell placed in a solution that is hypertonic to its contents will lose water

molecules by osmosis. The cytoplasm will cease to exert a pressure on the cellulose cell wall

and the cell, described as flaccid, will lack support. Water loss can continue to such an extent

that the cytoplasm, and attached cell membrane, contracts and detaches from the cell wall. A

cell in this condition is said to be plasmolysed and this damage is irreversible.

Safety notes

· Use care when working with glassware.

· Wash your hands before and after the lab.

· Use care when working around electrical light sources.

· Use care when using any chemicals in the lab.

· Care will be taken when using the scalpel.

· All laboratory surfaces will be kept clear and clean throughout the experiment.

Variables

The independent variable of this experiment is the molarity of the sodium chloride solution. This

will be changed and should cause changes in the dependant variables. The molarity of the

solution was chosen, as it will be relatively straightforward to mix different concentrations

successfully. The varying concentrations were worked out using a molarity table.

The dependant variables of this experiment are the changes in length and mass of the potato

cores, which should occur as a result of changing the independent variable. The length and mass

of the cores were chosen as dependant variables as changes can be visible using standard

laboratory equipment such as a top pan balance and a ruler.

Hypothesis

Pure water has the highest water potential, which is zero. If potato cores were placed into pure

water, the water potential inside the cells would be exceeded by the water potential of the

external solution, resulting in a net flow of water molecules into the cells by the process of

osmosis. This will be visible as an increase in length and mass of the potato cores.

It is predicted that as the solute potential of the external solution is decreased (i.e. the solution

becomes more concentrated) less and less water will move by osmosis into the cells and as a

result the increases in length and mass of the potato cores will be smaller. This will continue until

the isotonic point is reached, which is where the internal and external water potentials are equal,

and will be visible as no change in mass or length of the potato cores. After this point the solute

potential in the external solution will be less than that of inside the cell and therefore there will be

a net movement of water molecules out of the cell, resulting visibly in a decrease in length and

mass of the potato cores from their original size.

As osmosis is the diffusion of water molecules, and as diffusion is the random movement of

particles from areas of high concentration to low concentration, it might be expected that any

factors that speed up or slow down the movement of these particles will affect the rate of

osmosis.

It is predicted that the isotonic solutions, for both length and mass to remain unchanged, to be of

the same molarity.

Apparatus

· Potato tuber (large)

· 2 measuring cylinders

· 9 beakers

· Cork borer

· Ceramic tile

· Ruler

· Scalpel

· Top pan balance

· 27 boiling tubes

· 2 boiling tube racks

· Distilled water

· 1 molar sodium chloride solution

· Forceps

· Stopwatch

Method

· Mix up correct molar quantities of sodium chloride solution as shown in the molarity table as

below using a measuring cylinder, and place into beakers.

Molarity of Sodium Chloride (NaCL) Solution Volume of H20 / cm3 Volume of 1 molar NaCL

/ cm3

0.00 100 0

0.05 95 5

0.10 90 10

0.15 85 15

0.20 80 20

0.25 75 25

0.30 70 30

0.35 65 35

0.40 60 40

· Place correct quantity of pure distilled water into a beaker, measured using a different

measuring cylinder.

· Place all boiling tubes into boiling tube rack.

· Place 20cm3 of each solution into each of three separate boiling tubes. This will result in eight

sets of three test tubes, with each of the eight sets containing different molar concentrations of

sodium chloride ranging from 0.05molar to 0.40 molar.

· Place 20cm3 of pure distilled water into each of three separate boiling tubes.

· Cut 27 potato cores from the same large potato and place them onto a ceramic tile.

· Using a scalpel and ruler (calibrated in millimetres) cut the cores into 50mm lengths, with care

taken to ensure no potato peel being left on them. The cutting will be to an accuracy of 1

millimetre.

· The cores will then be individually weighed on a top pan balance to an accuracy of 0.01

grams.

· Each of the cores will then be placed, into one of the 27 boiling tubes for a duration of 2 hours.

· The timing will be done using a stopwatch.

· After 2 hours the cores will be removed and weighed directly on the top pan again to measure

changes in mass.

· Directly after weighing the cores, they will be measured again on a ceramic tile using a ruler in

order to note any changes in length.

· After the experiment has been completed all the apparatus will be properly placed away and

all the potato cores will be disposed of.

· All results will be recorded in a table as follows:

Concentration of NaCl solution/mol dm3 Initial Length or Mass of potato core (mm or grams)

Final Length or Mass of potato core (mm or grams) Change in Length or Mass of potato core

(mm or grams) Percentage change in Length or Mass/% Mean Percentage change in Length or

Mass/%

Fair Test

All variables, apart from the independent variable, must be kept constant in order to allow for a

fair test. These variables include:

· The temperature. This is because by increasing temperatures one is increasing the kinetic

energy of the molecules and as a result the diffusion rate will also increase.

· The length, mass and diameter of the potato cores, in order to allow for uniformity.

· The volume of solutions used, in order to allow for consistency.

· The time that the potato cores are left in the solution. This has to be kept constant as different

times of exposure to the sodium chloride solution will result in different amounts of osmosis

taking place.

· The same apparatus used, in order to allow for consistency.

· The same potato used for the core samples, in order to allow for consistency.

· The potato cores will only be handled with forceps in order to minimise contact with the cell

surface membranes.

Reliability

In order to conduct the experiment in as a reliable manner as possible, thereby diminishing the

chance of anomalous results occurring, the procedure of measuring the changes in length and

mass of potato cores in varying molar concentrations of sodium chloride will be repeated twice

for each concentration. This will also be the case with the control reading of distilled water. As a

result the mass and length of twenty-seven cores will be measured in total.

Analysis and Evaluation

The water potential of pure distilled water is zero, as there are no solutes present. It was in pure

water where the greatest increases in length and mass occurred (6.7% and 6.2% respectively),

due to the water potential inside the potato cells being far less than that of the water. This

caused a substantial influx of water molecules resulting in increases in length and mass.

The results confirm the hypothesis in that as the solute potential of the solution decreased (i.e.

the solution became more concentrated) the changes in length and mass of the potato cores

decreased. This was due to the difference in internal and external water potentials becoming

smaller.

The increases in length and mass of the potato cores meant that the cells were in various levels

of turgidity in the different molar concentrations. These increases in size continued to decrease

until the isotonic point was reached, where both the internal and external water potentials are the

same.

After the isotonic point has been reached, the cells initially begin to undergo slow plasmolysis

but this speeds up as the solute potential of the sodium chloride solution is further decreased and

the solution becomes more concentrated. This can be seen as a decreasing in length and mass

from the original.

Between 0.0 molar concentration and 0.10 molar concentration the changes in length appear to

be consistent as a straight line, thereby suggesting that increases in solute potential will result in

proportionally smaller increases in length. However between 0.0 molar concentration and 0.155

molar concentration the changes in mass appear to be inconsistent as the line is not straight but

disjointed at 0.05 molar, thereby suggesting that increases in solute potential will not necessarily

result in proportionally less increases in mass. This may have been the result of not allowing

excess solution on the external surface of the cores from draining away before placing it onto the

top pan balance. This superficial water would be measured as extra mass by the sensitive

weighing scales but would not be clearly visible when re-measuring the length of the cores.

The isotonic point was at 0.10 molar concentration for changes in length and 0.155 molar

concentration for changes in mass. This suggests that at a molar concentration of 0.1 there

appeared to be no changes in length of the core from the initial length, whilst there still appeared

to be an increase in mass occurring. This may have also been due to the excess weight of water

on the surface of the potato cores. This apparent error implies that the results demonstrating

mean percentage changes in mass of potato cores should have been decreased by a factor of

approximately 2%, thus accommodating the mistake, and so results would be more consistent

with those of changes in length.

Soon after the isotonic point the cells begin to show increasing signs of plasmolysis occurring.

Directly after 0.25 molar concentration of sodium chloride solution the decreases in length and

mass become quite rapid.

The maximum decrease in length and mass of potato cores occurs at 0.35 molar concentration.

However at 0.40 molar concentration there is an increase in mass and length.

The scenario does not state whether the flooded plants that have become flaccid in the 0.3

molar concentration could recover if placed back into optimal conditions, i.e. whether the cells

of the plant had become fully plasmolysed. This is where the connections between cells by

cytoplasmic strands called plasmodesmata are broken and the cell is non-recoverable. If the

cells have become fully plasmolysed then the plant cells are unable to cope with the low external

water potential. If the cells have not become fully plasmolysed then recovery could be possible

and the effects of osmosis have not been life-threatening for the plant in the short term.

Improvements

It would have been beneficial to have repeated the experiment more times to make certain that

the results were not gained through chance or by an external factor.

A greater range of molarities over smaller increments would have shown more accurately any

changes in length and masses of potato cores.

Ideally all samples should have come from the same part of the potato, as this would have

decreased the chances of variations in texture.

The size of the potato cores were more than likely to be inconsistent in shape as they were cut

by hand using a ruler for measurement. It may have been more appropriate to use a template of

some sort.

A variety of other similar plant roots could have also been placed through the same procedure in

order for comparison.

The experiment was also limited by the accuracy of the top pan balance, which was to one

decimal place, and the calibration of the ruler.

It was also unlikely that room temperature and pressure remained consistent throughout the

experiment conduction, and changes in temperature may have altered the rate of diffusion.

The potato cores should have had any excess water on their outer surfaces removed by blotting

with blotting paper before being re-measured, as this is likely to have altered the masses of the

cores.

Investigate the movement of water into and out of plant cells by osmosis.

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